<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        :root{background: green;}       
        .all p:empty{
            background: red;
            height:40px;
            /* clear: both; overflow: hidden; height:0; */
        }
    </style>
</head>
<body>
    <div class="all">
        <p>你好</p>
        <p></p>
    </div>
</body>
</html>